1. Technical Field
The present disclosure generally relates to HVAC systems and, more particularly, to methods and apparatus for controlling air handlers used at different altitudes.
2. Description of the Related Art
Modern structures, such as office buildings, manufacturing plants, barns, and residences, often are designed and constructed so as to allow control over environmental conditions within the structures. By way of example, environmental conditions such as temperature, humidity, air purity, air flow, enthalpy (combined value of temperature and humidity), and “fresh air” ventilation can be regulated to ensure that the interior environment of a structure is as may be desired for particular occupants and equipment housed in the structure, and for processes and procedures conducted within the structure.
In practice, one typical way of regulating environmental conditions within a building or other structure is by employing one or more duct systems to deliver processed air and, often, to remove air from within the building for processing. One common example of such a system is known as a heating, ventilating, and/or air conditioning (HVAC) system. HVAC systems generally are well-known, and a typical HVAC system can include, for example, components such as conduits (“ducts” or “duct systems”), air conditioners, compressors, heating elements, heat exchangers, filters, louvers (for controlling air flow to and from the exterior environment), blower fans, and airflow hoods. Simple HVAC systems can be designed employing a number of methods, including the equal friction method, the constant velocity method, the velocity reduction method, and the static regain method.
Optimizing the efficiency of an HVAC system can be viewed in terms of optimizing at least one desired HVAC system parameter to approach or equal a desired value. Predominantly, regulation of an environment within a structure entails controlling the aerial environment of the structure's interior which, in turn, usually requires measuring (and then controlling) one or more system parameters associated with the HVAC system being used. Thus, where the parameter of interest is temperature, air temperature can first be measured by using, for example, simple thermometers, thermocouples, thermistors, resistance thermometers, and more recently, solid state devices for computerized measurements. Temperature can then be controlled by passing the air in the HVAC duct system over, through, or near, one or more heating elements (if the air is too cold), or over or near a cool, compressed fluid (if the air is too warm), until the air reaches a desired temperature.
One variable impacting the efficiency of the HVAC system is the BTU content of the fuel used in the system, which is adjusted according to the elevation at which the system is installed. The natural gas supplied to higher elevations is “de-rated” to have a lower BTU content, thereby reducing the heating capacity of the system, to match the lower density of air that can be supplied for combustion. Many HVAC systems, however, use variable speed air handlers that attempt to maintain a constant mass flow of air regardless of air density, and therefore the air handlers may be operated at excessive speeds when used at higher elevations. The increased air handler speed may generate excessive noise in the system ductwork, promote formation of condensate on the heat exchangers thereby necessitating quicker replacement, and reduce the temperature of the air leaving the heat exchangers (which may deleterious impact performance of the system).
The reduced air density at higher elevations may also affect the accuracy of static pressure measurements provided for the system. Static pressure may be useful for properly installing, and then monitoring and maintaining, HVAC systems. Static pressure can be used to optimize system operation, and also as a diagnostic measure during system installation and service. Static pressure can be widely variable from installation to installation due, in part, to varied duct system configurations and blower systems, which are designed to accommodate a range of applications.
Additionally, static pressure calculations may also be impacted by air density. During HVAC control design, the static pressure of a system may be correlated to the speed at which the air handler is operated. When the system is installed at higher elevations, the air handler will operate at higher speeds to counter the drop in air density, thereby to maintain a constant mass flow of air. Conventional calculations, therefore, will predict higher values for static pressure than those that are actually observed.
It would thus be desirable to have controller methods and apparatus that correct for higher altitude installations when designing desired air handler speed and calculating static pressure of the system.